Method for solidifying smelt

ABSTRACT

A pair of horizontally-disposed water-cooled drums are mounted side-by-side so that the nip between the drums forms a reservoir for receiving molten smelt of the type which is derived from paper pulping processes. The nip gap is set between 0.005 and 0.025 inches, and drums are rotated downward through the reservoir while concurrently cooling the drums to reduce the localized temperature of the smelt at the nip to its freezing temperature and to remove the heat of fusion of the smelt. Accordingly a thin, brittle sheet of solidified smelt is passed between the drums and the sheet is free from the surface of either drum.

United States Patent Casten Oct. 14, 1975 METHOD FOR SOLIDIFYING SNIELT[75] Inventor: James W. Casten, Mountain Brook, Primary Exammerl zobertLmdsay Ala Attorney, Agent, or F zrm-Robert E. Krebs; Thomas S.

MacDonald [73] Assignee: Envirotech Corporation, Menlo Park, Calif. [57]ABSTRACT [22] Filed; O t, 23, 1973 A pair of horizontally-disposedwater-cooled drums are mounted side-by-side so that the nip between the[2-1] Appl' 408989 drums forms a reservoir for receiving molten smelt ofthe type which is derived from paper pulping pro- [52] US. Cl. 162/30;62/346; 264/175 CeSSeS- The nip gap s Set between 0.005 and 0.025 [51]Int. Cl. D21C 11/00 inches, and r m are rota e ownward through the [58]Field of Search 162/29, 30, 239, 240; rvoir hile conc rrently coolingthe drums to re- 62/346; 264/175 duce the localized temperature of thesmelt at the nip to its freezing temperature and to remove the heat of[56] Referen Cit d fusion of the smelt. Accordingly a thin, brittlesheet of UNITED STATES PATENTS solidified smelt is passed between thedrums and the t t 1,327,354 1 1920 Perry 264/175 x ghee free from heSurface of drum 2,590,544 3/1952 Kallok 264/ 175 X 3 Claims, 3 DrawingFigures U.S. Patent Oct. 14, 1975 METHOD FOR SOLIDIFYING SMELTBACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to an improved apparatus and method for transforming moltensalts into solid fragmemts and, more particularly, this inventionrelates to an apparatus and method for cooling hot liquid smelt of thetype which is derived from sodium-base or Kraft paper pulp liquors so asto form brittle, readily shattered sheets.

2. State of the Art In paper pulping processes, such as the Kraftprocess, it is common practice to burn the spent pulping liquor (i.e.,dilute black liquor in a fluidized bed reactor, a furnace or anincinerator. The residual, concentrated molten inorganic salts in suchfurnaces are known in the industry as smelt and generally comprises thefollowing salts: sodium sulfide (Na S), sodium sulfate (Na So sodiumhydroxide (NaOH) and sodium carbonate Na CO Small percentages of Na SOand Na S O may also be present.

A typical procedure for handling such smelt in the paper pulpingindustry comprises dropping the molten smelt into a water bath which isheld in a so-called dissolving tank. The resultant hot liquid solutionin the dissolving tank is known as green liquor and it can be recycledor returned to the pulping process. Specifically, the green liquorusually is recausticized to white liquor" by converting the sodiumcarbonate fraction to sodium hydroxide; the white liquor is then re-usedto digest raw materials, usually wood chips.

Molten smelt is generally at a temperature above I500 F. When it strikesthe water in a dissolving tank, the result is a noisy, immediate andpossibly explosive formation of steam and other vapors. Because of theextreme temperature difference between the water and the molten salt, afraction of the water vapor may be disassociated into elemental hydrogenand oxygen. To rid the plant of such potentially explosive vapors, largeblowers or fans are usually provided to sweep the vapors into a chimneyor stack.

Nevertheless, explosions and related accidents may occur. Such accidentshave, unfortunately, resulted in the loss of human lives. In fact, it isrecognized that black liquor boilers and the associated dissolving tanksconstitute on of the most dangerous areas in a paper pulp mill.Furthermore, the vapors from the dissolving tanks usually contain somecaustic materials and sulfur gases and, as such, are caustic andmaladorus.

To ameliorate such problems, it has been suggested to cool the smeltmaterials in a dry state. For example, it has been suggested to utilizemodifications of devices which are known as drum flakers to cool thesmelt. The operation of such drum flakers generally comprises applying apartially-cooled molten material in a thin layer to a revolving drumwhereon the material adheres, solidifies, and is thence removed in flakeform by a stationary knife. The stationary knife (or doctor blade) whichexfoliates-the solidified smelt from the drum surface is also a featureof a proposed process which utilizes a drum and a roller. In thatproposed process, hot smelt is fed onto the surface of the drum, isrolled by the roller to adhere the smelt to the drum and, finally, thesolidified smelt at a temperature of about 120 F. is scraped by a doctorblade from the drum as a thin peel (about 0.6 0.8mm thick).

OBJECTS OF THE INVENTION There is provided, in accordance with thisinvention, a novel and improved method and apparatus for transformingmolten smelt into solid fragments, where the smelt is of the type whichis derived from a paper pulping process. The inventive method andapparatus not only overcomes many of the disadvantages of proceduresproposed heretofore, but also provides significant advantages. Among theobjects and advantages of the present invention:

A primary object is to provide an improved highcapacity apparatus andmethod for solidifying molten smelt from a pulping process, where thesmelt generally comprises a mixture of sodium sulfide, sodium sulfate,and sodium carbonate;

A more specific object is to provide an apparatus and method forreducing such non-viscous molten smelt into solid fragments;

Another object is to form such molten smelt into brittle, readilyfragmented sheets; and

Still another object is to provide an apparatus that can accommodate arelatively uneven flow of molten smelt by providing a reservoir to holdand accommodate abnormally high momentary flows, such as arecharacteristic of recovery boiler operations.

SUMMARY OF THE INVENTION The method according to the present inventiongenerally comprises transforming pulping process smelt (which iscomprised, for example, of sodium or potassium sulfide, sulfate,hydroxide and carbonate) into substantially brittle sheets by extrusionbetween cold drums. Such extrusion is accomplished by drawing moltensmelt through the nip formed between two rotatably-mounted coolingdrums, whereby the sheet which leaves the nip is free of both drums,i.e., non-adhering, and, therefore, need not be scraped or otherwiseexfoliated from either drum. More specifically, the localizedtemperature of the smelt at the nip of the drums is lowered to the smeltfreezing temperature and, also, the heat of fusion of the smelt isremoved.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of thepresent invention may be readily ascertained by reference to thefollowing description and appended drawings, which are offered by way ofillustration only and not in limitation of the invention, whose scope isdefined by the appended claims and equivalents. In the drawings:

FIG. 1 is a side view of a device according to the invention;

FIG. 2 is an end view of the device of FIG. 1; and

FIG. 3 is a side view, partially schematic and drawn to an enlargedscale, of apparatus for use with the device of FIG. I and 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The device which isillustrated in FIGS. I and 2 generally comprises a pair ofhorizontally-disposed watercooled cylindrical drums 11 and 12 which aremounted side-by-side on a common base 15. The nip between thecylindrical surfaces forms an upper reservoir 17 into which molten smeltis supplied. The drums may be fabricated from various materials,including cast iron, steel, stainless steel or other castable orfabricated materials. The smelt is solidified by the cold drums and ispassed through the nip as a thin sheet as the drums rotate in oppositesenses downward through the smelt reservoir in the direction indicatedby the curved arrows in FIG. 2.

The drums are mounted for rotation with axially directed trunnions 20and 21 which are hollow and whose outer ends are supported on suitablejournal means 25 and 26 which are fixed to the base 15. Conventionaldrive means, such as an electric motor 29, are operatively coupled tothe trunnions to rotatably drive the drums at a preselected speed, saybetween eight and twenty revolutions per minute. The surface of thedrums should be smooth. In some instances, the drum surfaces may bechrome plated but that is generally not necessary.

Means are also provided to deliver cooling liquid, such as water, topreferably but not necessarily fill the interior of the drums. In theillustrated arrangement, liquid inlet conduits 34 are connected, viarotary collars 35, to the outer ends of the hollow trunnions 21. Alsoconnected to the rotary collars are liquid outlet conduits 37 whichremove warmed liquid from the drums. Similar drums are used in theprocess field as dryers; in such devices, steam is circulated in thedrying drums instead of cooling liquid as is used here. In the processaccording to this invention, the circulation of cooling liquid in thedrums should be sufficient to reduce the localized temperature of thesmelt at the nip to the smelt freezing temperature and to remove theheat of fusion of the smelt. In terms of the surface temperature of thedrums, the circulation of the coolant is preferably sufficient tomaintain the drum surface temperature below 300 F. although somewhathigher temperatures can be tolerated.

In the preferred embodiment, one of the rotating drums 12 is fixedlymounted with respect to the support frame and the other drum is movablymounted so that the minimum clearance at the nip between the drums canbe selectively adjusted.

In the illustrated arrangement, the nip clearance is determined by apair of pneumatic cylinders 41 connected to opposite ends of the drum12. More specifically, the journals 26 at each end of the movable drum12 are supported, respectively, by parallel plate members 38 whose lowerends are pivotably mounted at 39 to the supporting platform 15. Thepneumatic cylinders 41 are pivotably linked between the respective platemembers 38 and the support frame to urge the movable drum toward thefixed drum. The minimum clearance between the drums is determined bydraw bolts 42 which are connected between the plates 38 and the frame15. The pneumatic cylinders permit the nip clearance to expand toaccommodate, for example, variations in drum speed or in the rate offeed of the smelt. When the cylinders are extended, the support platesare pivoted and the drum 12 is thereby moved toward the stationary drum11 so far as the draw bolts 42 will allow. Other means could be utilizedto urge the drums together and to maintain the operating clearancebetween the drums; for example, springs could be attached between thedrums.

Experimentation has shown that the nip clearance between the drums isquite important. Generally speaking there must be enough space betweenthe drums so that a thin sheet of smelt will pass therebetween but notso much space as to allow the molten liquid to flow freely and escape.In practice, the preferred spacing at the nip between the drums is inthe range of 0.005 inches and 0.025 inches and the typical sheetthickness is between 0.005 and 0.025 inches.

It should be appreciated that the molten salts which are derived fromthe black liquor in paper pulping processes are very thin and ofrelatively low viscosity. Such salts typically melt in the l200- 1800 F.range and do not stick to a cool drum surface.

Black liquor smelt, as previously mentioned, generally comprises amixture of sodium sulfide, sodium sulfate, and sodium carbonate. Therelative percentages of these components can vary widely. A typicalblack liquor smelt may comprise, by weight, one-third Na one-sixth Na SOand one-half Na CO with the remainder being made up of other salts. Itshould be understood that the chemical composition of the smelt aftersolidification may differ from the presolidification composition becauseof oxidation and other reactions which occur during cooling.

To contain the molten feed in the nip area between the drums, a pair ofdam plates 46 are mounted at each end of the drums and are in slidingcontact with the drum heads. These dam plates may be fabricated, forexample, of steel or cast iron to withstand the high temperatures ofmolten smelt. It usually is advisable to extend the darn plates downwardpast the nip so that the smelt sheets which leave the nip are guideddownward.

In practice it may be desirable to mount light-duty, spring loadedmetallic wipers 49 below the nip of the drums as a safety device toremove traces of solidified smelt which may adhere to the drum surfaces.It should be emphasized, however, that such wiper blades are optionaland are significantly different from the doctor blades which are foundon conventional drum flaker devices. In those conventional devices, thedoctor blades act almost like chisels to exfoliate solidified smeltwhich has frozen to the drum surface, thereby to form flakes. In thepresent device, the wiper blades merely brush occasional bits of smeltor foreign materials from the drum surface.

The smelt which is processed according to this invention issues from thenip as a thin sheet or as a ribbon. Although the smelt is solidified,its temperature usually ranges from 600 to 800 F. Usually the sheetscool within a few feet after leaving the nip. The sheets themselves arequite brittle, and, as will be described hereinafter, it is relativelyeasy to shatter or fracture the sheets into small shards.

Illustrated in FIG. 3 is one configuration of a device for furthercooling and carrying the sheets of smelt away from the drums. Thatdevice generally comprises a covered, elongated slide or chute 51 whichextends at an incline from beneath the nip of the drums. The floor 53 ofthe chute is perforated so that air can flow therethrough. Anappropriate duct 55 communicates with the interior of the chute to carrycool air upward through the perforated floor 53. A fan can be used toinduce a draft so that the smelt sheets flow down the chute on a cushionof air.

At the bottom of the chute, a conventional conveyor 61, say of thepaddle-type, is provided to shatter the sheets into small fragments andthence to carry the shards to discharge. In some instances, a waterjacket 63 is provided about the lower half of the conveyor.

An experimental model of the previously described device has been builtand tested extensively. The

drums were watercooled, 12 inches in length, and 12 inches in diameter.The nip clearance was maintained by springs at an average of 0.025inches. The average thickness of the smelt sheets was 0.024 inches.Molten smelt at a temperature of about l600 F. was poured into the pinchto a pool depth of to 4 inches. The drums rotated at seventeenrevolutions per minute. It was calculated that the test device couldprocess about one thousand pounds of smelt per hour per foot of drumlength. The temperature of the solidified sheet leaving the pinch variedbetween 600 and 900 F. By was of comparison, devices which have beenproposed previously in this field would process only about 400 pounds ofsmelt per linear foot of drum per hour by using one drum to which theproduct is reported to be stuck and subsequently removed by a doctorknife.

I claim:

l. A process for forming sheets of solidified smelt from molten smeltsuch as the type which is derived above the nip between a pair ofclosely spaced drums having a minimum but expandable preset nipclearance of 0.005 to 0.025 inches;

b. rotating the drums into the liquid stream and resiliently urging oneof the drums toward the other;

c. concurrently cooling the drums to reduce the localized temperature ofthe smelt at the nip to its freezing temperature and to remove the heatof fusion of the smelt, and forming a thin, brittle sheet of solidifiedsmelt which issues at a temperature below about 900F vertically downwardfrom the nip and which is free of the surface of the either drum.

2. A process according to claim 1 wherein the temperature of thesolidified smelt which issues from the nip exceeds 300 F.

3. A process according to claim 1 wherein the smelt is admitted into thereservoir at a temperature ranging from the combustion of black liq lidi paper from l200-1800F and the solidified sheet of smelt is pulpingprocesses comprising:

a. admitting a stream of such molten liquid smelt at a temperature aboveabout 1200F into a reservoir issued from the nip at a temperatureranging from 600-800F.

1. A PROCESS FOR FORMING SHEETS OF SOLIDIFIED SMELT FROM MOLTEN SMELTSUCH AS THE TYPE WHICH IS DERIVED FROM THE COMBUSTION OF BLOCK LIQUORSOLIDS IN PAPER PULPING PROCESS COMPRISING: A. ADMITING A STREAM OF SUCHMOLTEN LIQUID SMELT AT A TEMPERATURE ABOVE ABOUT 1200*F INTO A RESERVIORABOVE THE NIP BETWEEN A PAIR OF CLOSELY SPACED DRUMS HAVING A MINIMUMBUT EXPANDABLE PRESET NIP CLEARANCE OF 0.005 TO 0.025 INCHES. B.ROTATING THE DRUMS INTO THE LIQUID STREAM AND RESILIENTLY URGING ONE OFTHE DRUMS TOWARDS THE OTHER, C. CONCURRENTLY COOLING THE DRUMS TO REDUCETHE LOCALIZED TEMPERATURE OF THE SMELT AT THE NIP TO ITS FREEZINGTEMPERATURE AND TO REMOVE THE HEAT OF FUSION OF THE SMELT, AND FORMING ATHIN, BRITTLE SHEET OF SOLIDIFIED SMELT WHICH ISSUES AT A TEMPERATUREBELOW ABOUT 900*F VERTICALLY DOWNWARD FROM THE NIP AND WHICH IS FREE OFTHE SURFACE OF THE EITHER DRUM.
 2. A process according to claim 1wherein the temperature of the solidified smelt which issues from thenip exceeds 300* F.
 3. A process according to claim 1 wherein the smeltis admitted into the reservoir at a temperature ranging from1200*-1800*F and the solidified sheet of smelt is issued from the nip ata temperature ranging from 600*-800*F.